1. Field of the Invention
The present invention relates to a quartz glass substrate for polysilicon thin film transistor liquid crystal display.
2. Discussion of Background
Quartz glass has the most excellent heat resistance among transparent glass materials. Further, the quartz glass has an extremely small thermal expansion coefficient, and it is excellent in dimensional stability. Furthermore, the quartz glass is excellent in chemical durability. Therefore, in these days it is used as substrate material for polysilicon thin film transistor (hereinafter referred to simply as "TFT") liquid crystal display (hereinafter referred to simply as "LCD"), particularly for small-size polysilicon TFT LCD which is used for viewfinder of video camera. Processes for producing polysilicon TFT can be generally classified into three large groups by level of manufacturing temperature: (1) high temperature process (the maximum process temperature: about 1,000.degree. C.), (2) medium temperature process (the maximum process temperature: about 700.degree. C.) and (3) low temperature process (the maximum process temperature: about 500.degree. C.).
Generally speaking, with respect to large area TFT such as television and display, developments in the direction of lowering the process temperature are widely proceeding from the viewpoint of manufacturing cost. On the other hand, with respect to small-size polysilicon TFT for viewfinder of video camera, etc., if a high temperature process is applied, a conventional LSI manufacturing line can be utilized without big modification, whereby highly reliable process technology developed in manufacturing LSI can be effectively used and a high quality TFT can be produced with a high yield. Thus, a high temperature process is dominant.
A problem here is heat resistance of glass used as substrate. From this viewpoint, quartz glass is usually used as substrate. As practical evaluation method for heat resistance, there is a method wherein deformation degree of a shape in actual use is measured and evaluated. (For example, a substrate having a predetermined thickness, width and length is placed on a jig having a predetermined span and subjected to heat history of a predetermined temperature and time, whereby deformation in warp is measured.) Although this method is simple, it is difficult to make quantitative evaluation as properties of material. On the other hand, as a method for evaluating heat resistance as properties of materials, there is a method wherein evaluation is made in accordance with temperature dependency of viscosity of a material. Since viscosity deformation is dominant with respect to heat resistance of quartz glass at least 1,000.degree. C., this evaluation method is appropriate as evaluation method for properties of materials. Further, so far as quartz glass is concerned, a slope of temperature dependency of viscosity is considered to be substantially constant within a range of from 1,000 to 1,200.degree. C. Thus, a temperature at which a viscosity is a certain value, for example, a temperature at which the viscosity is 10.sup.13 poise (annealing point), can be applied as index. As specific measurement method, a conventional method of beam bending method can be applied.
Heat resistance of quartz glass is considerably different depending on kinds and manufacturing processes. Among quartz glass, so-called fused quartz glass having an annealing point of about from 1,170.degree. to 1,220.degree. C., is the most excellent in heat resistance.
Whereas, a substrate comprising synthetic quartz glass is far excellent in quality, for example, in view of bubbles and foreign substances contained in the glass, while an annealing point of synthetic quartz glass is generally about from 1,050.degree. C. to 1,120.degree. C., which is lower by about from 100.degree. C. to 150.degree. C. than fused quartz glass. Accordingly, there have been problems in a substrate comprising synthetic quartz glass when used for polysilicon TFT because of lower heat resistance. Further, it has been known that quartz glass usually has structural defects by which absorption in a wavelength of from 230 to 300 nm occurs with irradiation of ultraviolet under vacuum such as plasma etching applied in TFT manufacturing step. Among these structural defects, there is a case wherein fluorescent emission is observed, whereby such a practical problem arises that color tone is injured when such glass is used for color liquid crystal display elements.
When quartz glass is used for such optical use, optical homogeneity is required. As factors controlling this optical homogeneity (which is usually indicated by variation of refractive index .DELTA.n), 1 thermal distortion and 2 change of composition exist. Thermal distortion can be removed by annealing glass sufficiently. However, if an element which causes to change refractive index, is contained in glass, it is difficult to obtain high homogeneity because of the element. For example, G. Hetherington et al. (Physics and Chemistry of Glasses 3 (1962) No. 4, p129) have reported that OH contained in quartz glass gives the quartz glass a variation of about 1.times.10.sup.-6 /10 ppmOH. Likewise, it is reported that Cl contained in quartz glass gives the quartz glass a variation about 1.times.10.sup.-6 /10 ppmCl (Susa et al. (Journal of non-crystalline solid 79 (1986) p165-176)). Accordingly, to attain such homogeneity that a variation of refractive index is not more than 5.times.10.sup.-6, it is readily considered that variation range of the OH and Cl contents is reduced and the OH and Cl contents are uniformly distributed throughout the quartz glass. However, for quartz glass which is produced by a process wherein a glass-forming raw material is subjected to flame hydrolysis to form a porous glass product, followed by vitrification, or quartz glass which is produced by a process wherein a glass-forming raw material is subjected to heat hydrolysis and quartz glass is directly deposited on a substrate, it is difficult to uniform the distribution of OH throughout the quartz glass due to influence of e.g. temperature distribution when produced.
A method which is considered next is to reduce an absolute amount of OH contained in quartz glass. It is known that an amount of OH in quartz glass produced by a process wherein a porous quartz glass product is subjected to vitrification, is reduced by treating the porous glass product with a halogen compound such as Cl.sub.2, CCl.sub.4, NH.sub.4 F or HF, as means for producing optical fibers having a low transmission loss. However, the glass produced by the process has a halogen element distributed therein, which brings about a variation of refractive index.
Furthermore, Japanese Unexamined Patent Publication No. 102139/1990 discloses a method to improve homogeneity with a downwardly convex distribution of the OH concentration in the quartz glass. According to this method, in order to form the downwardly convex OH distribution in the quartz glass, it is required to precisely control thermal treatment conditions. Further, the method disclosed in Japanese Unexamined Patent Publication No. 102139/1990 can not be substantially applied to quartz glass produced by way of a porous quartz glass product by a vapor phase reaction, because an upwardly convex OH distribution is formed in the quartz glass.
It is an object of the present invention to solve the above problems and to provide a polysilicon TFT LCD quartz glass substrate excellent in heat resistance and suitable for manufacture of polysilicon TFT LCD which is used for viewfinder, etc. It is another object of the present invention to solve the above problems and to provide a polysilicon TFT LCD quartz glass substrate which is substantially absent from fluorescent emission.